Hypothesis

Core idea

With advancing age, cytosolic cystatin B loses its inhibitory potency through oxidative modification of its active‑site cysteine, whereas lysosomal cystatin B remains functional due to the protective acidic lumen and lumenal chaperones. This selective loss creates a compartment‑specific defect: leaked cathepsins remain unchecked in the cytoplasm, damaging mitochondria and triggering neuroinflammation, while intra‑lysosomal proteolysis (including AGE clearance) stays intact.

Mechanistic rationale

• Cystatin B contains a reactive cysteine (Cys73) that forms disulfide bonds or S‑nitrosylation under oxidative stress, reducing its affinity for cathepsins B, L and D.
• The cytosol of aged neurons exhibits elevated ROS and RNS, favoring such modifications.
• Lysosomal lumen maintains a low pH (~4.5) and high concentration of lumenal thiol‑oxidizing enzymes that keep cystatin B in a reduced state, preserving its inhibitory loop.
• Oxidized cystatin B cannot form the tight canonically‑binding interaction with cathepsins, allowing cytosolic cathepsins to cleave substrates such as mitochondrial proteins or NLRP3, amplifying pyroptosis.
• Because lysosomal cathepsin activity depends on low pH rather than cystatin B, intra‑lysosomal degradation proceeds normally, explaining why cathepsin D over‑expression still improves AGE clearance.

Testable predictions

1. Biochemical – Cytosolic extracts from old rat brain will show higher levels of S‑nitrosylated or disulfide‑linked cystatin B compared with young extracts; lysosomal fractions will not.
2. Genetic – Neuronal expression of a cysteine‑to‑serine mutant of cystatin B (C73S) that resists oxidation will reduce cytoplasmic cathepsin activity and neuroinflammatory markers in aged mice.
3. Pharmacological – Treatment with a cell‑permeable thiol‑reducing agent (e.g., N‑acetylcysteine) will restore cytosolic cystatin B inhibition and decrease cathepsin‑B‑dependent mitochondrial damage without altering lysosomal AGE degradation.
4. Imaging – Proximity ligation assays will detect decreased cystatin B–cathepsin B complexes in the cytosol of aged neurons, while lysosomal colocalization remains unchanged.

Falsifiability

If oxidative modification of cystatin B does not increase with age, or if rescuing cystatin B redox state fails to curb cytoplasmic cathepsin toxicity, the hypothesis is refuted. Likewise, if lysosomal cystatin B activity declines in parallel with cytosolic loss, the compartment‑specific protection model collapses.

Connection to SENS

This mechanism fits SENS damage category 5 (intracellular junk) by proposing a repairable protein‑level lesion: age‑dependent oxidation of cystatin B. Restoring its redox state constitutes a true repair rather than mere enhancement of degradation, aligning with the SENS goal of fixing the underlying molecular damage.

References (inline)

• Cathepsin D elevation in aged brain: https://pubmed.ncbi.nlm.nih.gov/8157122/
• Selective cytosolic translocation without lysosomal rupture: https://pubmed.ncbi.nlm.nih.gov/16914181/
• Cathepsin‑processed amyloid‑like fibrils that permeabilize lysosomes: https://pmc.ncbi.nlm.nih.gov/articles/PMC12871838/
• Impaired AGE degradation in photoaged fibroblasts: https://pubmed.ncbi.nlm.nih.gov/29501392/
• Cytoplasmic cathepsin B drives microglial neuroinflammation: https://pubmed.ncbi.nlm.nih.gov/41271184/